Posted to Technical Theory Forum on 5/31/2014
Vacuum and compression waveforms
Well I took a little time to experiment with vacuum and
compression waveforms on my personal car. I used a homemade
vacuum transducer using radio shack parts, a Pico pressure
transducer and a PV-350 for the Verus screen shots. I was
wondering if I could tell it the valves needed adjusting or
not. So I took a compression waveform and a vacuum waveform
while doing different things with the valve adjustment.
First I tightened the valve lash to zero on #4 Cylinder
exhaust valve. This is what I got: [2000 Honda Accord EX,
Engine/Propulsion Waveform]. Notice the pressure spike in
the intake when the intake opened and the valve overlap.
I then wondered what it would look like with all the exhaust
valves tight. This is What I got: [2000 Honda Accord EX,
Engine/Propulsion Waveform]. Notice the 5 volt swing of
the Vacuum transducer. Also notice the higher running
compression reading of a little over 90 psi. Extra exhaust
volume raised the pressures.
I then tightened the intake and exhaust valves to zero lash.
This is what I got: [2000 Honda Accord EX,
Engine/Propulsion Waveform]. Notice it had about a 5 volt
swing of the Vacuum transducer. The engine would barely idle
with the valve like that. The running compression was about
I then Adjusted the valves to spec. This is what I got:
[2000 Honda Accord EX, Engine/Propulsion Waveform]. Notice
that the vacuum transducer only had about a 3 volt swing.
Running compression was only about 65 psi.
If all the valves are the same out of spec, the only way you
will know for certain there is a problem is if you have know
good vacuum waveforms. If one valve is bad with adjustment
or leaking I think you would be able to see it and with an
overlay to determine which cylinder.
I thought it would be great on the Honda V6 engine to see if
it needs a valve adjustment when they have misfire codes but
they run good in the shop. I can upload the whole file if
someone wants to look at it.
Robert from California
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